Joint devices are typically used for connecting two cable ends in an extruded cable system. Such cables typically comprise a conductor and an insulation system surrounding the conductor. The insulation system may include inner and outer semiconducting layers for screening, and an intermediate electrical insulation layer arranged between the semiconducting layers.
Two types of joints are common, namely factory joints and prefabricated joints. Often they may also be referred to as sea (factory) and land (prefabricated) joints. This relates to the transport situation on land, where short cable sections are transported to the laying site on drums, and many joints need to be installed on site. Boats transporting sea cables to a site may load more than a hundred kilometers, and the required jointing can be done under factory conditions, prior to loading and transport.
Contrary to a factory joint, where the insulation material is applied directly onto the conductor, a prefabricated joint is mounted onto the insulation system of the cable. A prefabricated joint may include a field control body which includes at least one semiconducting layer and an electrical insulation layer which control the electric field, and a connector that completes the metallic link between the two cable conductors for current transfer.
FIG. 1 shows a standard connector 1 which forms part of a prefabricated joint, and two cable ends provided with respective conductors 5a, 5b, and electrical insulation systems 3a, 3b. The conductors 5a and 5b are generally attached to the connector 1 by means of respective screws 7a and 7b. In the ideal case the connector 1 is well centred, as shown in FIG. 1. This minimises the pressure difference around the cable circumference.
The screws 7a, 7b may typically be arranged on one side of the connector 1, pushing down the conductors 5a, 5b towards the inner surface of the connector 1. However, due to (a) the conductor not being solid, and (b) the inner radius of the connector for installation purposes being larger than the conductor radius, the connector 1 will not end up in a centred position shown in FIG. 1 once the screws 7a and 7b have been tightened, as illustrated in FIGS. 2a and 2b. The deformed conductor 5a will be pressed towards the opposite side of the connecter 1 and, as an unintended consequence the connector 1 will be pushed upwards, creating a bigger step between the connector and the electrical insulation system 3a, 3b surface on the screw side, than on the opposite side.
If the step dtop between the connector 1 and electrical insulation system 3a, 3b is large enough, a relatively large gap length Igap between the two materials will form compared to the situation when the connector 1 would not be pressed downwards due to the screws, as shown in FIGS. 3a and 3b. This gap may be formed between a deflector 9 of the joint, which controls the field distribution inside the joint, the connector 1 and the electrical insulation system 3a, 3b. Moreover, the existence of a step implies a pressure loss towards the semiconducting layer that surrounds the connector 1. The section, which has an axial length here referred to as Ipressure loss, subjected to this pressure loss is even longer than the gap length Igap. The risk of electrical breakdown is increased as soon as the pressure is lower than a nominal pressure, meaning that the length of the section with increased breakdown risk is even longer than the length of the gap length Igap.